Known in the art is an internal combustion engine which is designed to be able to change a mechanical compression ratio by a variable compression ratio mechanism, which is provided with a detection sensor able to detect the mechanical compression ratio, and which uses an output signal of the detection sensor as the basis to control the mechanical compression ratio to a target compression ratio (see Japanese Patent Publication (A) No. 2006-161583). In this internal combustion engine, a mechanical compression ratio control system is employed whereby if a signal line which conveys the output signal of the detection sensor breaks, the mechanical compression ratio becomes a maximum mechanical compression ratio.
However, when the engine load is high, if the mechanical compression ratio is raised, a combustion end temperature in the combustion chamber will become higher and as a result problems will arise such as the occurrence of pre-ignition or knocking. Therefore, in the above-mentioned internal combustion engine, to suppress the occurrence of knocking when a break in the signal line causes the mechanical compression ratio to become the maximum mechanical compression ratio, the amount of intake air is reduced and the ignition timing is delayed. That is, the amount of intake air is reduced to prevent a high load operating state, while the ignition timing is delayed so as to suppress a rise in the combustion pressure.
In this regard, it may be possible to suppress the occurrence of knocking in this way by reducing the amount of intake air and delaying the ignition timing because the maximum mechanical compression ratio is not that high. However, if making the maximum mechanical compression ratio a further higher compression ratio, when the mechanical compression ratio is the maximum mechanical compression ratio, even if reducing the amount of intake air or delaying the ignition timing, it will become difficult to block the occurrence of knocking. That is, in this case, if a signal line breaks or another abnormality occurs, insofar as employing a mechanical compression ratio control system where the mechanical compression ratio becomes the maximum mechanical compression ratio, it will be difficult to prevent the occurrence of pre-ignition or knocking.